If you're confident that everything has been done, put your Beeb back together and switch on...success!
If the BREAK key appears to have given up on you at this point, the likelihood is that the keyboard connector is resting on the new 8271 chip. Switch off again and make sure that the keyboard connector is making contact with all the pins.
If the keyboard is misbehaving generally or you get a continuous tone and nothing else, check that you haven't shifted the keyboard connector along a pin or two when you plugged it back in. This is the Beeb equivalent of buttoning your cardigan up wrong and comes to us all with age.